I have got doubts in understanding this paragraph written by Norman Liberman.

Ethane being lighter component than propane, when it reaches top vapor along with Propane, the dew point Temperature of the mixture gets reduced.

I just didn't understand, when column top temperature set value is still same as before, why does reflux ratio gets decreased and inturn cause butane to get through the top tray vapors ?

Kindly guide.

3.7. Effect of Lighter Components on Overhead Composition Control I noticed on my depropanizer in Texas City that the butane content of my propane product was erratic, even though the tower top pressure and temperature were constant. However, when I studied the lab data closely, I noticed an interesting trend. When the ethane content of the propane product was high, so was the butane. As the ethane content of the depropanizer feed became erratic, so did the butane content of the propane. But why? The vapor leaving the top of a tower is at its dew-point temperature and pressure. As the ethane content of a propane vapor stream flowing at its dew point goes up, the dew-point temperature will decrease. The ethane makes the propane more volatile. If the reflux rate is on tower-top temperature control, the lower dew-point temperature will reduce the reflux to restore the temperature set point. The reduced reflux flow permits more butane to be distilled overhead. The resulting variable butane content of the overhead product reflects not so much a malfunction of the temperature control, but a natural variation in the overhead ethane content. What I eventually did on my depropanizer was to integrate the online gas chromatograph (GC) to reset the tower-top temperature set point for ethane and butane composition. Now, with advances in computer control, this technique has become common